Conventional semicrawler-type working vehicles are known, for example, from Japanese Patent Application H11-321729 and U.S. Pat. No. 6,199,646 B1. Each working vehicle has a track frame that pivotably supporting a semicrawler unit relative to the vehicle body by means of a pivot shaft, a drive wheel arranged on a drive shaft, a front driven wheel and a rear driven wheel supported on the track frame such that a horizontal distance from the drive wheel to the front driven wheel is longer than a horizontal distance from the drive wheel to the rear driven wheel, and a crawler belt wound around the drive wheel, front driven wheel and rear driven wheel. The pivot shaft for pivotably supporting the track frame has its axis positioned on a vertical line that runs through an axis of the drive wheel. And, a tension is applied to the rubber crawler belt by causing a forward protruding movement of the front driven wheel with a tension adjusting mechanism.
In such a conventional semicrawler-type working vehicle, an amount of rearward protrusion of the semicrawler unit from the drive wheel is shortened to avoid an interference with any work machine that is mounted to the rear of the tractor, and an amount of forward protrusion of the semicrawler unit from the drive wheel is lengthened to increase a traction force by increasing a ground-contacting length of the crawler belt. This structural characteristic is retained for different field conditions such as wet fields (paddy fields) and dry fields.
Incidentally, in such a semicrawler-type working vehicle, when lifting the entire vehicle body including the front wheels and the semicrawler units without any tension being applied to the crawler belt, the track frame tends to assume a slight front-down tilted posture since it has the tension adjusting mechanism in its front portion. When a tension is applied to the crawler belt in this state, a centripetal force acting in an inward direction of the semicrawler unit is produced in the drive wheel and the front and rear driven wheels, since the crawler belt is being pulled circumferentially.
When the centripetal force affecting the drive wheel and the front and rear driven wheels is compensated, the resultant force becomes approximately zero and the track frame does not move. When the horizontal distance from the drive wheel to the front driven wheel and the horizontal distance from the drive wheel to the rear driven wheel are equal while in contact with the ground (forming an isosceles triangle in which the portion of the crawler belt in contact with the ground is the base), at the time when the track frame transitions from its natural, slight forward tilting posture to a horizontal posture, the forces acting on the drive wheel and the front and rear driven wheels compose directly below the axis of the drive wheel and the resultant force becomes substantially zero, thus resulting in a balanced state.
When the horizontal distance from the drive wheel to the front driven wheel is set longer than the horizontal distance from the drive wheel to the rear driven wheel while in contact with the ground, a larger torque is produced by the application of tension in the span from the drive wheel to the front driven wheel in comparison to that produced in the span from the drive wheel to the rear driven wheel, since the former span is longer. Then, in order to neutralize the resultant force, a posture alteration is made so that the slight front-down tilted posture of the track frame becomes horizontal and into a front-up posture (a slight rear-down tilted posture in which the front portion of the track frame is slight is raised).
And, although the track frames become substantially horizontal to the ground when the semicrawler units contact the ground while in this slight rear-down tilted posture, a front-raising force (prestress) is applied to the semicrawler units.
This front-raising force works as a floating direction force in soft ground areas such as wet fields (paddy fields) and has the function of preventing the semicrawler units from sinking. However, this results in reduced traction force when working on dry fields or the like.
An object of the present invention is to provide a working vehicle equipped with semicrawler units that are improved to better suit wet fields and dry fields.